Abstract. Humidity climatologies of the tropical uppermost troposphere from satellite
limb emission measurements have been compared. Four instruments are
considered; UARS-MLS, Odin-SMR, and Aura-MLS operating in the microwave
region, and MIPAS in the infrared region. A reference for the comparison is
obtained by MOZAIC in-situ measurements.

The upper tropospheric humidity products were compared on basis of their
empirical probability density functions and seasonally averaged horizontal
fields at two altitude layers, 12 and 15 km. The probability density
functions of the microwave datasets were found to be in very good agreement
with each other, and were also consistent with MOZAIC. The average seasonal
humidities differ with less than 10%RHi between the instruments, indicating
that stated measurement accuracies of 20–30% are conservative estimates.
The systematic uncertainty in Odin-SMR data due to cloud correction was also
independently estimated to be 10%RHi. MIPAS humidity profiles were found to
suffer from cloud contamination, with only 30% of the measurements reaching
into the upper troposphere, but under clear-sky conditions there is a good
agreement between MIPAS, Odin-SMR and Aura-MLS.

Odin-SMR and the two MLS datasets can be treated as independent, being based
on different underlying spectroscopy and technology. The good agreement
between the microwave limb-sounders, and MOZAIC, is therefore an important
step towards understanding the upper tropospheric humidity. The found
accuracy of 10%RHi is approaching the level required to validate climate
modelling of the upper troposphere humidity. The comparison of microwave and
infrared also stresses that microwave limb-sounding is necessary for a complete
view of the upper troposphere.